The present invention relates to a silyl ketal functionality incorporated into a polymeric material, radiation-sensitive compositions containing this functionalized polymeric material, and methods of using these radiation-sensitive compositions to produce patterns. More particularly, the invention relates to the use of silyl ketal protected, base soluble polymers in chemically amplified resist compositions and methods for using these resist compositions in lithographic imaging.
Lithography, which is the patterning of radiation-sensitive polymeric films with a source of activating energy, including high-energy radiation sources such as photons, electrons, or ion beams, is a principle means of defining high-resolution circuitry in the manufacture of semiconductor devices. These radiation-sensitive films, or photoresists, generally consist of multiple component formulations that are usually spin-coated onto a desired semiconducting substrate such as, for example, a silicon wafer, and are patternwise imaged with radiation. The radiation is most commonly ultraviolet light of wavelengths 248 nanometer (nm), 193 nm, or 157 nm, a beam of electrons or ions, or soft x-ray radiation, also known as extreme ultraviolet (EUV) at a wavelength of approximately 13 nm. This radiation induces a chemical transformation that renders the solubility of the exposed regions different from that of the unexposed regions in a chosen resist developer, most commonly an aqueous solution of tetramethylammonium hydroxide or another aqueous alkali solution.
The most commonly employed of these resist formulations in high-resolution imaging (e.g., features less than 250 nm in width) are of a class of materials termed chemically amplified, in which the aforementioned radiation-induced chemical transformation is catalytic in nature and thus results in multiple chemical reactions for each photon, ion or electron absorbed by the film. Chemically amplified resists (CARs) allow for high-resolution, high-contrast and high-sensitivity that are not afforded by other resists. Positive tone CARs are typically composed of an intrinsically aqueous alkali-soluble polymer, such as poly(4-hydroxystyrene), poly(norbornenecarboxylic acid, poly(acrylic acid), poly(methacrylic acid), and similar structures, as well as copolymers, terpolymers, or higher order polymers containing these repeat units, that have been rendered insoluble by the partial protection of its solubilizing moieties with acid labile protecting groups. These protecting groups are cleaved by strong acid that is produced within the film by the exposure of photoacid generators (PAGs)xe2x80x94compounds that have been formulated with the polymer in the resist composition.
The protecting groups are often classified by their activation energy, or the amount of energy required to be supplied to the system after the formation of strong acid that will result in a suitable degree of deprotection within the resist polymers to render a change in their aqueous base solubility. Protecting groups that require an activation energy in excess of 30 kcal/mol are often termed high activation energy resists. Examples of protecting groups that are in this category are tertiary butyl esters and isopropyl esters of carboxylic acids. Protecting groups with activation energies less than 25 kcal/mol are often referred to as low activation energy resists, for example acetals. Additionally, other protecting groups, such as tertiary butyl carbonates, fall into an intermediate range of activation energies (e.g., 25 to 30 kcal/mol) and are thus classified as mid-activation energy resists. The energy required to overcome the activation barrier and to allow deprotection is most often supplied by post-exposure baking (PEB) of the resist films.
Conventionally, low activation energy protection groups, such as ketals, acetals, or silyl ethers have been incorporated into polymeric materials for the proposed use as imaging materials for the production of electronic devices, as described, for example, in U.S. Pat. Nos. 5,712,078, 6,037,097 and 6,043,003. These materials have some attractive advantages over high activation energy systems. Most notably, the low activation energy resist comprised of ketal protected poly(4-hydroxystyrene) known as KRS (ketal resist system), as described, for example, in U.S. Pat. Nos. 6,043,003 and 6,037,097, tend to be far less sensitive to the effects of post-exposure delay (PED).
PED, which refers to the time between exposure and subsequent processing (e.g., post-exposure baking, etc.) of a resist film, has been shown to result in line width variation and/or poisoning of the resist film by prolonged exposure to certain contaminants (e.g., atmospheric contaminants) that in turn results in deleterious acid neutralization at the film-air interface and yields structures with unacceptable profiles. As the deprotection reaction in many low activation energy CARs occurs at or near ambient temperature, (e.g., 20 to 25 degrees Celsius (xc2x0 C.)), these KRS resist films are not susceptible to PED. Furthermore, the KRS photoresists have been shown to produce films that are stable under proper storage conditions to upward of 30 days, are independent of PEB-induced line width variations (e.g.,  less than 1 nm/xc2x0 C.) over a temperature range of from 80xc2x0 C. to 120xc2x0 C., and can be successfully processed without employing PEB. These features make the KRS resists particularly attractive for e-beam exposures, either for use in e-beam projection lithography (EPL) for semiconductor device manufacture or with direct-write exposure systems in the production of photolithography masks.
In order for the semiconductor industry to progress to sub-100 nm features, next generation lithography (NGL) options are being developed. Despite the differences in the various NGL strategies such as EPL, EUV, or 157 nm optical lithography, all will require the use of thin resist films to accommodate the mechanical stability necessary in printing high-resolution features. Currently used resist film thickness (e.g., typically 300 nm to 1000 nm) would result in aspect ratios in excess of 3.0 for sub-100 nm images. It has been shown that aspect ratios greater than about 3.5 result in image collapse during post-development aqueous rinsing. Furthermore, thinner resists are required for improved resolution and depth-of-focus. However, by employing thinner resists, the effectiveness of the resist as an etch barrier is significantly diminished.
One method for imparting increased etch resistance to conventional resist materials is the incorporation of organometallic species in the resist composition. Examples of this include the covalent attachment of silicon, germanium and tin containing moieties into the polymeric structure of the resist matrix, or the blending of small molecule and/or polymeric organometallic materials. This technique allows for nonvolatile oxides, halides, and/or oxyhalides to form during the etch process when the plasma employed contain oxygen, halogens, or haloalkane (e.g., fluorocarbon) chemistries. However, resist materials formed using this conventional approach are subjected to post-exposure delay and the aforementioned disadvantages associated therewith (e.g., resist poisoning, etc.).
Thus, there exits a need for high-resolution, stable resist compositions having increased etch resistance and that can be employed in lithographic imaging (i.e., lithographic patterning) techniques and the like.
It is an object of the present invention to provide a chemically amplified resist composition having improved etch resistance and hydrolytic stability while maintaining high resolution and robust process latitude which can be employed, for example, in nanoscale lithographic patterning.
It is another object of the present invention to use a cycloaliphatic silyl ketal as an acid labile protecting group for derivatizing intrinsically base soluble polymers, and utilizing such polymers in chemically amplified resist systems.
Advantageously, the present invention addresses the need for a high resolution, stable resist composition having increased etch resistance by combining a low activation energy protecting group and an organometallic species into the same chemical functionality, namely, a silyl ketal. Specifically, the present invention provides a multicomponent chemically amplified resist prepared from, among other constituents, an initially base soluble polymeric material that has been at least partially protected with a silyl ketal group, but may also contain other protecting groups as well as unprotected acidic functionalities, the ratio of which is selected to most effectively modulate a solubility of the resist composition in an aqueous base or other developer. Furthermore, the novel use of a silyl ketal functionality as a protecting group for polymeric acidic (e.g., acid dissociation constant pKa less than 14) oxygen containing moieties provides desirable absorption characteristics of the resist composition, specifically for 157 nm optical lithography, due to an intrinsic transparency of this functional group at a wavelength of around 157 nm.
A resist composition, in accordance with one aspect of the invention, comprises an aqueous base soluble polymer or copolymer having one or more polar functional groups, wherein at least one of the functional groups is protected with a cycloaliphatic silyl ketal group but may also include other protecting groups as well as unprotected acidic functionalities. The resist composition further comprises an acid generator, preferably a photoacid generator (PAG), and a casting solvent, and may also include a base and/or a surfactant.
In accordance with another aspect of the invention, a method of patterning a desired substrate, such as, for example, a silicon wafer, a chrome-on-glass mask blank, or a printed circuit board, is provided. The method may include the following steps: applying a coating of resist composition containing an inherently aqueous base soluble polymeric material that is at least partially protected with a cycloaliphatic silyl ketal group to the desired substrate; patternwise exposing the resist film to an imaging radiation source; developing and removing exposed areas of the resist film; etching into the substrate in the exposed areas using an etching process; and removing any remaining resist from the substrate, for example, using a stripping agent.
In accordance with another embodiment of the invention, the resist compositions described herein may be employed as an imaging layer in a bilayer resist system, preferably by coupling the resist composition with an organic underlayer composition. In this illustrative embodiment, the resist composition is not applied directly to the substrate but is applied to an organic underlayer which is applied to the substrate. The method may include the following steps: applying an organic underlayer composition to the substrate; applying an imaging layer to the underlayer, the imaging layer comprising a resist composition containing a polymeric material that is at least partially protected with a silyl ketal group; patternwise exposing the resist composition to an imaging radiation source; developing and removing areas of the resist composition exposed to the imaging radiation source; etching into the underlayer in the exposed areas; etching into the substrate in areas exposed by the underlayer etching step; and removing any remaining resist composition and underlayer composition from the substrate.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.